This invention relates to compounds which have a mesomorphic phase of the nematic liquid crystalline type. Materials having a mesomorphic phase do not pass directly from the solid crystalline state to the liquid isotropic state upon being heated, but rather have an intermediate state in which the material has some of the physical properties of a liquid and some of the optical properties of a crystal within their mesomorphic temperature ranges.
It should be recognized that externally applied energy, such as an electric potential, strong magnetic field, heat, ultrasonic vibrations, and even mechanical vibrations, causes a change in the molecular orientation and/or movement which results in changes in optical properties. Such changes include, but are not limited to, birefringence, light diffusion or scattering, light dispersion, or a combination thereof.
There are several mesomorphic phases or forms, mainly, the nematic smectic and cholesteric mesophases. The term "nematic" means "thread-like" and is applied to those materials which, when in their mesomorphic stage assume a characteristic thread-like texture in which the molecules or aggregates thereof have one axis parallel. This is the lowest degree of order of molecular arrangement of the several mesomorphic phases, the smectic phase being of the next higher order of molecular arrangement. The cholesteric mesophase is a special case of the nematic phase.
The present invention is concerned with a new family of thermotropic nematic compounds. While there are numerous known thermotropic nematic compounds, such as 4,4' di-n-heptoxyazoxybenzene, p-azoxy anisole, and anisylidene p-aminophenylacetate, these materials as a group have the drawback that their nematic mesophases occur at temperatures well above room temperature. As a result their utility is limited to applications within their elevated nematic temperature ranges. Moreover, such compounds have relatively high viscosities when in the nematic range and may have sufficient color to mask the change in optical properties. In addition, many of the known nematic compounds are expensive to produce and difficult to synthesize.
A few homologues outside the defined group of compounds of the present invention have been disclosed in the prior art, but these prior art homologues either exhibit no nematic phase or exhibit a nematic phase starting at relatively high temperatures (e.g. about 85.degree. C.). Reference may be made to Zeit. Phys. Chem., vol. 53B, pp. 75-84 (1942); J. Chem. Soc., pp. 4616-4617 (1954) and pp. 4305-4306 (1955); .[.and Chem. Abstracts, v. 69, col. 43200(g) (1968)..]. .Iadd.Chem. Abstracts, vol. 69, pp. 4039 Ref. 43200g (1968). .Iaddend.
The following compound, falling outside the definition of the novel compounds of the present invention, was found to be unsuitable for the purposes of the present invention: N-(p-ethoxybenzylidene)-p-methyl aniline.